These results should not come as a surprise nor would I call the binder use idiosyncratic. As someone who uses medieval techniques, I know that different pigments work best with different types and ratios of binder, and that it is common to add a little egg yolk to red pigments to improve their handling qualities and gloss.

Interesting point, Rachel – the use of egg yolk with red pigments specifically is noted in historic treatises as well. What is interesting about the ‘Praying Prophet’ and the other illuminations in its parent volume is that the yolk is not used in combination with any specific pigment, but rather with all pigments when used to decorate specific parts of the page, i.e. the scenes within the initials. The very same pigments, when used in the decorative borders, do not show the presence of absorption bands characteristic of egg yolk.